With the Sun beating down on the northern hemisphere, it’s hard to realize that we’re farthest from the Sun for the entire year today. We’re about a million and a half miles farther than the average distance of 93 million miles.

That distance forms one of the basic “yardsticks” for all of astronomy. It’s called the AU -- the astronomical unit. Instead of expressing distances within the solar system in miles or kilometers, astronomers use the AU. So Earth is one AU from the Sun, Mars is about one and a half, mighty Jupiter is more than five, and Pluto averages almost 40 AU.

Determining the length of the astronomical unit took a lot of work. Some of the earliest estimates were made using the size of Earth’s shadow during a lunar eclipse, the angle between the Sun and Moon in the sky, or the length of shadows cast at different locations at the same moment. These measurements were so hard to get right, though, that estimates were off by tens of millions of miles.

Starting in the 1600s, astronomers used transits of Venus across the face of the Sun. Recording the time and angle of a transit from different places on Earth made it possible to calculate the Sun’s distance. But that was tough to do as well, so the measurements still weren’t quite right.

It wasn’t until the 20th century -- using radar and signals from spacecraft -- that astronomers finally zeroed in on the true length of the AU -- the yardstick of the solar system.